The present invention relates to the field of computer ink-jet printers.
With the advent of computers came the need for devices which could produce the results of computer generated work product in a printed form. Early devices used for this purpose were simple modifications of the then current electric typewriter technology. But these devices could not produce picture graphics, nor could they produce multicolored images, nor could they print as rapidly as was desired.
Numerous advances have been made in the field. Notable among these has been the development of the impact dot matrix printer. While that type of printer is still widely used, it is neither as fast nor as durable as required in many applications. Nor can it easily produce high definition color printouts. The development of the thermal ink-jet printer has solved many of these problems. U.S. Pat. No. 4,728,963, issued to S. O. Rasmussen et al., and assigned to the same assignee as is this application, describes an example of this type of printer technology.
Thermal ink-jet printers operate by employing a plurality of resistor elements to expel droplets of ink through an associated plurality of nozzles. In particular, each resistor element, which is typically a pad of resistive material about 50 .mu.m by 50 .mu.m in size, is located in a chamber filled with ink supplied from an ink reservoir comprising an ink-jet cartridge. A nozzle plate, comprising a plurality of nozzles, or openings, with each nozzle associated with a resistor element, defines a part of the chamber. Upon the energizing of a particular resistor element, a droplet of ink is expelled by droplet vaporization through the nozzle toward the print medium, whether paper, fabric, or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column on the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath, the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
In order to obtain multicolored printing, a plurality of ink-jet cartridges, each having a chamber holding a different color of ink from the other cartridges, may be supported on the printhead.
Current ink-jet technology printers are not able to print high density plots on plain paper without suffering two major drawbacks: the saturated media is transformed into an unacceptably wavy or cockled sheet; and adjacent colors tend to run or bleed into one another. The ink used in thermal ink-jet printing is of liquid base. When the liquid ink is deposited on wood-based papers, it absorbs into the cellulose fibers and causes the fibers to swell. As the cellulose fibers swell, they generate localized expansions, which, in turn, causes the paper to warp uncontrollably in these regions. This phenomenon is called paper cockle. This can cause a degradation of print quality due to uncontrolled pen-to-paper spacing, and can also cause the printed output to have a low quality appearance due to the wrinkled paper.
Hardware solutions to these problems have been attempted. Heating elements have been used to dry the ink rapidly after it is printed. But this has helped only to reduce smearing that occurs after printing. Prior art heating elements have not been effective to reduce the problems of ink migration that occur during printing and in the first few fractions of a second after printing.
Other types of printer technology have been developed to produce high definition print at high speed, but these are much more expensive to construct and to operate, and thus they are priced out of the range of most applications in which thermal ink-jet printers may be utilized.
The user who is unwilling to accept the poor quality must either print at a painfully slow speed or use a specially coated medium which costs substantially more than plain paper or plain medium. Using specially coated media, satisfactory print quality can be achieved at print resolutions on the order of 180 dots per inch. However, the problems such as ink bleeding are exacerbated by higher print resolution. In particular, it has heretofore not been possible to achieve acceptable color printing or throughput on plain paper medium at 180 dots per inch.
Using thermal transfer printer technology, good quality high density plots can be achieved at somewhat reduced speeds. Unfortunately, due to their complexity, these printers cost roughly two to three times as much as thermal ink-jet types. Another drawback of thermal transfer is inflexibility. Ink or dye is supplied on film which is thermally transferred to the print medium. Currently, one sheet of film is used for each print regardless of the density. This makes the cost per page unnecessarily high for lower density plots. The problem is compounded when multiple colors are used.
It is therefore an object of the present invention to provide a color ink-jet printer which prints color images on plain paper which are comparable in quality to color images printed on special papers.
A further object is to provide a plain paper color ink-jet printer characterized by high throughput and reliable, quiet operation.